i) Field of the Invention
The invention relates to pulp furnish having a mineral filler content from 50 to 90%, by weight, based on total solids, for papermaking; paper sheet having a filler content from 40 to 90%, by weight; and process of making filled paper from the pulp furnish.
ii) Description of the Prior Art
The paper, paperboard and plastic industries produce rigid and flexible sheets for a large variety of uses. The plastic sheets are normally more flexible, tear resistant and stretchable, and more dense and slippery than paper sheets, while common base paper sheets are normally more porous and much less water resistant. For purposes of handling and printing thereon, paper sheets are normally much more attractive than plastic sheets. In order to impart the plastic sheet with some characteristics of paper the addition of mineral fillers is required. The incorporation of inorganic fillers into thermoplastic polymers has been widely practiced in industry to extend them and to enhance certain properties, namely opacity and brightness, and also to lower the material cost. U.S. Pat. No. 6,054,218 describes a method to produce a sheet made of plastic material and inorganic filler which feels like and has at least some of the properties of paper. The filled plastic sheet according to the invention comprises a multilayer structure having an outer layer, a middle layer, and an inner layer. The layers comprise different proportions of polyethylene, filler namely calcium carbonate, and pigments namely titanium dioxide and silicate adapted to give a feel of paper to the multilayer sheet.
The process to produce the filled plastic paper comprises the co-extrusion and calendaring steps of a thermoplastic polymer such as polyethylene and inorganic fillers and pigments at a temperature higher than the melting point of the thermoplastic polymer, which can be as high as 200 deg. C. A product of this nature has been manufactured by A. Schulman Inc. and marketed under the trademark Papermatch®. The manufacturer claims that the process can be used for manufacturing packaging applications, and for labels, envelopes, wall paper, folders and a variety of other products. Natural Source Printing, Inc. at present commercializes FiberStone® Paper, which is also designated as stone paper or rock paper. According to published sources of this company the stone paper made from polyethylene combined with up to 80% calcium carbonate fillers can be employed as a substitute for traditional papers used in the printing industry, such as synthetic paper and film, premium coated paper, recycled paper, PVC sheet, labels, and tags. Being impervious to water the stone paper can also be very useful for outdoor applications.
While the above stone papers have the advantages of being made without the use of ligo-cellulose fibres and water, they present some major drawbacks: high amounts of petroleum oil-based polymers, high density and low stiffness. They can be neither recycled, nor biodegradable. The analysis of some commercial stone papers revealed that the sheets are multilayered structures with 54 to 75% inorganic material and the rest is thermoplastic polymer namely high density polyethylene (HDPE) and coating material. Depending on the level of inorganic material used with thermoplastic the density of sheets is in the range of 0.9-1.4 g/cm3. In order to achieve the required values of opacity, bulk, stiffness and strength the sheets have to be made with high basis weights (200 to 300 g/m2 or more.) The basis weight or grammage is the weight per unit area of sheet. Bulk is a term used to indicate volume or thickness in relation to weight. It is the reciprocal of density (weight per unit volume). It is calculated from calliper and basis weight of sheet: Bulk (cm3/g)=Calliper (mm)*Basis Weight (g/m2)*1000. Decrease in sheet bulk or in other words increase in density makes the sheet smoother, glossier, less opaque, and lower in stiffness. Yet, in many applications, such as those used in copy printers, the most critical property is the stiffness of sheet, which is heavily reduced as the density is increased.
Because of the general disadvantages of the plastic-based stone paper described above, there is a need to produce super-filled sheets from renewable, recyclable, biodegradable and sustainable materials and using the conventional papermaking process. The super-filled sheets must also have low density and the required bulk, opacity, and strength properties even when they are produced at basis weights half of those commercially available plastic-based stone paper sheets. Normal printing fine papers made with filler contents up to 28% have specific densities ranging between 0.5 and 0.7 g/cm3, which are almost half of the plastic-based stone papers. For some applications the super-filled sheets need to have water resistant characteristics.
Inorganic (mineral) fillers are commonly used in manufacturing of printing papers (copy, inkjet, flexo, offset, gravure) from aqueous dispersions of wood pulp fibers to improve brightness and opacity, and achieve improvements in sheet print definition and dimensional stability. The term “fine” paper is used in the conventional industry sense and includes tablet, bond, offset, coated printing papers, text and cover stock, coated publication paper, book paper and cotton paper. The offset fine paper is surface sized with a formulation mainly composed of starch and hydrophobic polymer, such as styrene maleic anhydride, after the paper web has been dried. The internal filler levels in normal fine papers may range from 10 to 28%. As fine paper suitable for offset and gravure printing must have sufficient strength to withstand the high speed printing operation, it has been found that the existing papermaking technologies are not suitable to make them with a filler level higher than 30%.
Paperboard base sheets are made up of one or more fibrous layers or plies and generally with no filler addition. Depending on the end-use; paperboards are classified as: 1) carton board (various compositions used to make folding boxboard and set-up/rigid boxes); 2) food packaging board (used for food and liquid packaging); and 3) corrugated board (used for containers consisting of two or more linerboard grades separated by corrugated medium glued to the liners). Depending on application, the surface finish of the product is often obtained by single or double coating using known formulations which may be composed of inorganic fillers and pigments, binders and barrier polymers. Some packaging grades have their surfaces covered by polymeric films to impart high barrier properties to gas, water vapour or liquids. Paperboard base sheets are made almost exclusively from virgin and recycled fibres and additives. For some white toped multiply grades a very limited amount of inorganic filler (around 5%) is sometime introduced to the top ply sheet to improve opacity and print quality.
Making paper or paper board with high internal filler levels similar to those of plastic-based stone paper and having the required properties could be a means for making low cost green products for a variety of applications namely printing papers, flexible packaging, labels, tags, maps, bags, wall papers and other applications. The cost of papermaking fillers, such as precipitated calcium carbonate (PCC), ground calcium carbonate (GCC), kaolin clay, talc, precipitated calcium sulphate (PCS) or calcium sulphate (CS), is generally lower than the cost of cellulose fibres. The savings for the papermaker to produce one ton of paper can be substantial if the filler can be used to replace large quantities of expensive purchased kraft fibres. Because filled paper web is much easier to dry than paper web made with no filler, drying energy is lower. Since high filler addition will substantially improve the opacity of sheet, it might be possible to obtain this desired property at lower basis weight. Moreover, a filled base paper requires less coating material to achieve the required quality of normal coated grades.
The common method of introducing filler to paper sheet is by metering the filler slurry to a pulp suspension of about 1 to 3% consistency at locations such as in a machine chest or at the inlet of the fan pump, prior to the head box of the papermachine. The filler particles normally have a similar negative charge to that of fibres and thus have little propensity to adsorb onto the fibre surfaces. As a result, retention of filler particles with pulp fibres during sheet making is difficult to achieve, especially on high speed modern paper machines where furnish components experience large shear forces. Therefore, a polymeric retention aid system is always added to the diluted papermaking furnish, prior to the headbox of the papermachine, to enhance filler retention by the known agglomeration and flocculation mechanisms. However, with the existing retention aid technologies, achieving high filler retention without impairing sheet formation or structural uniformity is still a major challenge. For example, on a modern fine paper machine running at a speed of 1400 m/min, first-pass filler retention is about 40-50%. This means that only about half of the amount of filler in the furnish is retained in the sheet during its formation and the remaining portion drains with process water, which is often referred to by the term white water. In many mills paper machine runnability problems, high sewer losses of filler, holes in sheet and increased cost of functional additives (sizing, optical brightener, starch), have been associated with poor filler retention and accumulation of filler in the white water system.
In the art of papermaking once the moist web is formed it will requires adequate wet-web strength for good runnability on the paper machine. The dry sheet will require high Z-direction strength, tensile strength and stiffness for runnability on printing presses and copiers, and for other end uses. It is well known that the major obstacle to raising filler content in printing grades to higher levels is limited by the deterioration of these strength properties. Because filler does not have bonding capacity, inclusion of filler in paper impedes fibre-fibre bonding. On adding filler to sheet, tensile strength and elastic modulus are inevitably reduced by replacement of fibres by filler particles; not only are there fewer fibres in the sheet, which reduces the strength of fibre-fibre bonds, but also the presence of filler reduces the area of contact and prevents intimate bonds from occurring between fibres. As a result, filler addition drastically reduces wet web strength. A wet paper containing a high amount of filler can break more easily at the open draws of a paper machine. Therefore, strong wet web is an important criterion for good paper machine runnability. Fillers are denser than fibres and thus their addition will also reduce sheet bulk, which is essential for bending stiffness. Poor bonding of filler particles in the fibrous structure can also increase surface dusting in offset printing.
It is well known that the strength of paper sheet is affected by the length and surface area of fibres which influences the relative bonded area in the fibre network. The bonded area can be increased by fibre refining and by the web consolidation in the press section of the paper machine. Increasing bonding area by pressing and fibre refining can increase the internal bond strength and tensile strength of sheet, but at the expense of its bulk. At a given basis weight a decrease in sheet bulk may reduce bending stiffness. However, despite these possible negative effects on bulk and stiffness, in recent years good fibre development by refining and better forming and pressing techniques have improved the strength of filled sheets, and most fine paper manufacturers have now the possibility to increase filler contents in their grades by a few percent points [“Practical ways forward to achieving higher filler content in paper”, C. F. Baker and B. Nazir, Use of Minerals in Papermaking, Pira Conference, Manchester February 1997)].
Another well known method to increase paper strength, but without changing the density of the sheet, is the addition of natural and synthetic polymers. They are commonly added in small proportions, which may range from 1 to 20 kg/ton of paper, to the aqueous pulp furnish, or applied on the sheet surface after the paper web has been dried. The performance of cationic strength polymers is often low when added to long fibre furnish such as kraft fibre because of its low negative charge and area of surface available for adsorption of the polymers. The performance can be completely impaired when cationic polymers are introduced to aqueous pulp furnishes having unfavourable chemistry conditions, such as high levels of anionic dissolved and colloidal substances and high conductivity.
Despite the progress in papermaking techniques and chemistries, the current filler content in all uncoated fine paper sheets is often below 30% of the paper weight. By using the conventional technologies, attempts to increase the filler content of these grades to higher levels result in insufficient filler retention, wet-web strength, tensile strength, and stiffness, and lower surface strength. An adequate surface strength is required for preventing dusting and linting when running on a high speed printing press, namely during offset printing.
In recent years several patents have been granted for making highly filled papers. U.S. Pat. No. 4,445,970 teaches a method to make printing fine paper suitable for offset and gravure printing at high speeds and containing high filler levels for a wide range of basis weights. High filler levels were achieved with high basis weight sheets, e.g., over 120 g/m2. These highly-filled fine papers were produced on a low speed Fourdrinier paper machine from a furnish containing large quantities of filler, preferably a mixture of clay and talc, and including 3-7% of an cationic latex which is selected to provide good retention and good strength without leaving a residue on the screen. Fine paper sheet of 120 g/m2 made by this invention with 46% filler has a tensile strength of 0.665 km. This tensile strength is considered to be very low when compared with a normal fine paper of 73 g/m2 made with 20% filler which has a tensile strength of about 6.0 km. Despite the addition of very high dosage rates of cationic latex the filler content in paper achieved by the invention of this patent U.S. Pat. No. 4,445,970 is still below 50%.
A number of prior patents disclose the general idea that strength of paper can be increased by addition of cationic latex to the paper-making furnish. Because of the basic electro-chemical properties of anionic furnish components, cationic latex interacts with fibre surfaces to provide additional fiber bonding and, accordingly, strength to the resultant paper. These patents relate primarily to so-called “high-strength” papers which are largely devoid of fillers, or at best contain only very small quantities of fillers. For example, U.S. Pat. No. 4,178,205 Wessling et al discusses the use of cationic latex, but pigment is not essential. U.S. Pat. No. 4,187,142 Pickleman et al discloses the use of an anionic polymer co-additive with cationic latex, with the use of a sufficient amount of latex to make the entire paper-making system cationic; the use of fillers is not mentioned in any example. Foster et al U.S. Pat. No. 4,189,345 discusses extremely high levels of cationic latex.
U.S. Pat. No. 4,181,567 Riddell et al relates to the manufacture of paper using an agglomerate of ionic polymer and relatively large quantities of filler. The patentees indicate that either anionic or cationic polymers may be used, and fillers mentioned are calcium carbonate, clay, talc, titanium dioxide and mixtures. In example 1, an 80 g/m2 basis weight paper having 29% filler is produced using calcium carbonate as the filler. This patent in essence discusses precipitation of the pigment with a retention aid system prior to its addition to the furnish composition.
It has been known in the paper Industry that the addition of anionic latex to the wet end of a paper machine combined with cationic chemical, such as alum, causes the anionic latex to precipitate in the presence of the fibers and fillers and thereby gives the paper increased strength. This procedure is normally used in the manufacture of certain so-called “high-strength” products such as gasket material, saturated paperboard, roofing felt, flooring felt, etc. No similar technique has heretofore been suggested for the manufacture of paper sheets having quantities of filler up to 90%.
It has been proposed noting U.S. Pat. No. 4,225,383 McReynolds in the manufacture of relatively thick paper product, similarly to the manufacture of roofing and flooring felt papers, to use the combination of a cationic polymer with anionic latex, and substantial quantities of mineral filler. However, the product is not designed for printing papers, and its strength requirements are accordingly relatively low. Moreover, because of the substantial heaviness of the paper produced by such a technique, the additional strength is originated merely by means of its mass.
Several other patents, including, U.S. Pat. No. 4,115,187, U.S. Pat. No. 5,514,212, GB 2,016,498, U.S. Pat. No. 4,710,270, and GB 1,505,641, describe the benefits of filler treatment with additives on retention and sheet properties. It is known that since most common inorganic filler particles in suspension carry a negative charge, the cationic additive adsorbs on their surfaces by electrostatic interactions causing them to agglomerate or flocculate. For anionic additives to promote flocculation the filler particles would require a positive charge to allow adsorption of the anionic additive. The aggregation of filler particles improves retention during sheet making and can also decrease the negative effect of filler on sheet strength, but excessive filler aggregation can impair paper uniformity and also decrease the gain in optical properties expected from the filler addition. The filler content achieved by these patents is below 40%.
In U.S. Pat. No. 7,074,845 Laleg anionic latex has been used in combination with swollen starch for preparing treated filler slurries to be added internally in paper manufacture. The swollen starch/latex compositions are prepared by pre-mixing latex with slurry of starch granules in a batch or jet cooker, or by adding hot water to the mixture under controlled conditions in order to make the starch granules swell sufficiently to improve their properties as a filler additive but avoid excess swelling leading to their rupture. The anionic latex interacts with cationic swollen starch granules forming an active matrix. The composition is rapidly mixed with the filler slurry, which increased filler aggregation. The treated filler is then added to the papermaking furnish prior to sheet making. The retention of treated filler prepared by this process, in the web during papermaking was improved and the filled sheets have a higher internal bond and tensile strength than filled sheets produced using the conventional addition of cooked starch to the furnish.
International Publication Number WO 2008/148204 Laleg et al discusses a method to increase strength of filled paper sheet by continuous treatment of filler slurry to enhance the fixation of anionic latex on precipitated calcium carbonate particles in a short time. In this process anionic latex is added to filler slurry at ambient temperature and then mixed with water having a temperature higher than the glass transition temperature (Tg) of the latex used. To efficiently fix the latex the temperature of the filler/latex mixture must be 20-60° C. higher than the Tg of the latex used. The anionic latexes applied by this process are totally and irreversibly fixed or bound onto the filler particles and the aggregated filler slurry is stable over time. In this invention the latex-treated filler slurry is designed for addition to papermaking furnishes at any point prior to the headbox of the paper machine or stored for later use. The latex-treated filler slurry improved filler retention, greatly prevented loss of sheet strength and improved performance of internal sizing agents.
In U.S. Pat. No. 5,824,364, calcium carbonate crystals are disclosed as being directly formed onto fibre fibrils by a precipitation procedure of calcium hydroxide and carbon dioxide without addition of fixing agents. The calcium carbonate filler contained in the sheet is limited to the available surface area of the fibre fibrils, as specified by the inventors, in the range of 3-200 m2/g. The objective of this prior art method was to achieve high filler retention by focusing on individual sections of the fibres, such as in the lumen, cell wall, or fibrils. The filler content in paper achieved by this invention was below 30%. In this patent no latex or other chemical agents were used to assist filler fixation on fibrils surface and to improve bonding.
FI 100729 (CA 2,223,955) discloses filler for use in papermaking, the filler comprising porous aggregates formed from calcium carbonate particles deposited on the surface of fines. According to the patent specification, this filler of a novel type is characterized in that the fines are made up of fine fibrils prepared by beating cellulose fibre from chemical or mechanical pulping. The size distribution of the fines fraction mainly corresponds to wire screen fraction P100. The paper filler content reached by this approach or by a similar approach described in U.S. Pat. No. 5,824,364 and US 2003/0051837 was around 30% and the strength properties were only slightly higher than those measured on sheets produced by conventional methods of filler addition.
While the above methods are claimed to help produce sheets having high filler content and with acceptable strength, any attempt to raise the filler to high levels up to 50% or more has never been made on a conventional paper machine or commercially. Poor filler retention, weak wet web and dry strength and low paper stiffness remain as major obstacles for papermakers. Obviously there is still a need for a technology to fabricate super filled pulp fibrous sheets without the papermaking problems mentioned above. It would be very useful if a simple composition could be conceived to permit fixing large portions of filler particles on fibrous surfaces and act as glue or binder and load bearing transfer between the materials that form the final paper product. It would be more practical, for some applications, if the final product has some barrier and water resistance characteristics.